Various systems have been proposed to supply an optimal air-fuel mixture to an internal combustion engine in accordance with the mode of engine operation, one of which is to utilize the concept of an electronic closed loop control system based on a sensed concentration of a component in exhaust gases of the engine.
According to the conventional system, an exhaust gase sensor, such as an oxygen analyzer, is deposited in an exhaust pipe for sensing a concentration of a component of exhaust gases from an internal combustion engine, generating an electrical signal representative of the sensed component. A differential signal generator is connected to the sensor for generating an electrical signal representative of a differential between the signal from the sensor and a reference signal. The reference signal is previously determined in due consideration of, for example, an optimum ratio of an air-fuel mixture to the engine for maximizing the efficiency of both the engine and an exhaust gas refining means. A so-called proportional-integral (p-i) controller is connected to the differential signal generator, receiving the signal therefrom. A pulse generator is connected to the p-i controller, generating a train of pulses which is fed to an air-fuel ratio regulating means, such as electromagnetic valves, for supplying an air-fuel mixture with an optimum air-fuel ratio to the engine.
In the previously described control system, a problem has been encountered that the output of the exhaust gas sensor falls to a considerable extent at a low ambient temperature, resulting in the fact that the feedback control of the system can be no longer carried out properly due to, for example, disturbance of external noises. In the above, the reason why the output of the sensor falls under such a condition is that internal impedance of the sensor rises with decrease of an ambient temperature. Furthermore, in general, at cold engine start, in order to secure good engine start and stable engine running operation, it is necessary to supply the engine with a rich air-fuel mixture. Such a rich mixture, however, can not be supplied to the engine at cold engine start through the feedback control. In order to remove this defect, it might be proposed by those skilled in the art that the system should be modified in a manner to start the feedback control when the output of the exhaust gas sensor exceeds a reference voltage, and, whilst, to terminate the feedback control when the output of the exhaust gas sensor falls below the above mentioned reference voltage.
However, in spite of the above proposal, another problem is encountered which results from the fact that the same reference voltage determines both the start and the termination of the feedback control. More specifically, after starting the engine, when the output of the exhaust gas sensor increases with warming up of the engine, it is desirable that the feedback control should be started as soon as possible. On the other hand, when the output of the exhaust gas sensor decreases with lowering of the engine temperature after stopping a vehicle, the feedback control, on the contrary, should be terminated as soon as possible. This is because the lowering of the output of the exhaust gas sensor makes the air-fuel mixture richer, resulting in air pollution due to noxious components in exhaust gases and lessening fuel economy. Therefore, it is understood that a reference voltage starting the feedback control should be less than that terminating the same.
It is therefore an object of the present invention to provide an improved electronic closed loop control system for removing the above described inherent defects of the prior art.
Another object of the present invention is to provide an improved electronic closed loop air-fuel ratio control system which changes a reference voltage in order to cause the feedback control to start or terminate at different voltage levels of the exhaust gas sensor's output.